Doping is an operation which consists in incorporating atoms or molecules in a material in order to modify the properties of the material. For example, in the field of optical fibers, dopants are incorporated in silica in order to modify its refractive index. The dopant can then be germanium if it is desired to increase the refractive index of the silica, or fluorine if it is desired on the contrary to lower the index.
The silica used can be natural silica or synthetic silica. Nevertheless, in the field of optical fibers, it is synthetic silica that is used most often. Synthetic silica is silica that is obtained by chemical synthesis, e.g. by oxidation of a silica-precursor gas in the presence of heat, for example silicon tetrachloride SiCl4. That reaction leads to a powder that is very pure with a grain size that is very fine, i.e. a grain size lying in the range 0.1 nanometers (nm) to 100 nm, and as a result the powder has a high specific surface area. Such a silica powder is known as “soot”.
Silica soot can be used, for example, to fabricate a preform by the method of vapor axial deposition (VAD) or outside vapor deposition (OVD).
Document JP 62252335 describes a method of fabricating a preform in which the silicon compound is hydrolyzed in the presence of silicon oxifluoride, thus leading to a deposit of fluorine-containing silica soot which is subsequently vitrified.
Those methods are well known to the person skilled in the art of optical fibers, and they are not described in greater detail below.
Silica soot can also be transformed using the method described in document EP 0 578 553. The resulting silica grains can then be deposited and vitrified in order to increase the diameter of primary preforms manufactured by the modified chemical vapor deposition (MCVD) method.
In order to manufacture those silica grains, the particles of soot are agglomerated by a sol-gel method so as to form granules, and the granules are then densified by heating, which enables the pores that exist between the various particles making them up to be eliminated so that the resulting grains are dense. In general, such grains are of a size that is greater than 1 micron (μm). The term “silica granule” is used to designate a porous particle of silica at an intermediate stage in the fabrication of densified silica grains.
It is possible to perform the operation of densifying granules under an atmosphere containing a gas that is a precursor of the desired dopant. Thus, in order to fluorinate silica granules, densification is performed under an atmosphere containing a fluorine-containing gas such as sulfur hexafluoride SF6 or silicon tetrafluoride SiF4. The granules of silica are placed in a crucible which is in turn placed in an oven so as to raise it to the temperature that enables densification to take place, the oven being fed with a gas that is a precursor of the desired dopant. Doping takes place by diffusion and reaction of the dopant molecules in the silica granules, thereby leading to the formation of complex molecules of the SiO2−xF2x type. The method is performed at high temperature, i.e. around 1400° C.
An alternative device enabling moving granules to be densified in the presence of a fluorine-containing gas is described in FR 2 749 005.
Those methods give rise to a certain number of problems.
The first drawback lies in the aggressivity of fluorine-containing gases. The high corrosivity of the fluorine-containing gases that are used at high temperature gives rise to massive corrosion of the ovens, thereby leading to high maintenance costs. Another problem is the handling of such gases and their treatment or recovery that is required because of their toxicity. Finally, these gases, and in particular SiF4, are of non-negligible costs.
Their second drawback lies in the lack of uniformity of the doping that is achieved in this way, which lack of uniformity is associated with the way the fluorine-containing reagent diffuses within the powder being treated, particularly when the powder is deposited in a crucible.